Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Solar cells typically consist of a p-n semiconducting junction. FIG. 1 is a cross-sectional view of an interdigitated back contact (IBC) solar cell. In the IBC solar cell, the p-n junction is on the back or non-illuminated surface of the solar cell. Photons 10 enter the solar cell 100 through the top (or illuminated) surface, as signified by the arrows. These photons pass through an anti-reflective coating 104, designed to maximize the number of photons that penetrate the substrate 100 and minimize those that are reflected away from the substrate. The ARC may be comprised of an SiNx layer. Beneath the ARC 104 may be a SiO2 layer 103, also known as a passivation layer. Of course, other dielectrics may be used. On the back side of the solar cell 100 is an emitter region 204.
Internally, the solar cell 100 is formed so as to have a p-n junction. This junction is shown as being substantially parallel to the top surface of the solar cell 100, although there are other implementations where the junction may not be parallel to the surface. In some embodiments, the solar cell 100 is fabricated using an n-type substrate 101. The photons 10 enter the solar cell 100 through the n+ doped region, also known as the front surface field (FSF) 102. The photons with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material's valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction. Thus, any e-h pairs that are generated in the depletion region of the p-n junction get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons are absorbed in near surface regions of the device, the minority carriers generated in the emitter need to diffuse to the depletion region and get swept across to the other side.
As a result of the charge separation caused by the presence of this p-n junction, the extra carriers (electrons and holes) generated by the photons can then be used to drive an external load to complete the circuit.
The doping pattern is alternating p-type and n-type dopant regions in this particular embodiment. The n+ back surface field 204 may be between approximately 0.1-0.7 mm in width and doped with phosphorus or other n-type dopants. The p+ emitter 203 may be between approximately 0.5-3 mm in width and doped with boron or other p-type dopants. This doping may enable the p-n junction in the IBC solar cell to function or have increased efficiency. FIG. 6 shows a commonly used pattern for the back side of the IBC solar cell. The metallic contacts or fingers 220 are all located on the bottom surface of the solar cell 100. Certain portions of the bottom surface may be implanted with p-type dopants to create emitters 203. Other portions are implanted with n-type dopants to create more negatively biased back surface field 204. The back surface is coated with a passivating layer 210 to enhance the reflectivity of the back surface. Metal fingers 220b are attached to the emitter 203 and fingers 220a attaches to the BSF 204.
Thus, to form the IBC solar cell, two patterned doping steps may be required. These patterned doping steps need to be aligned to prevent the p+ emitter 203 and the n+ back surface field 204 from overlapping. Poor alignment or overlapping may be prevented by leaving a gap between the p+ emitter 203 and the n+ back surface field 204, but this may degrade performance of the IBC solar cell. Even when properly aligned, such patterned doping may have large manufacturing costs. For example, photolithography or hard masks (such as an oxide) may be used, but both are expensive and require extra process steps.
FIG. 2 is a block diagram of a first method to form an IBC solar cell according to the prior art. This process requires two patterned diffusion steps (shown as “Screen Print Patterned Resist”) which must be well aligned to produce the pattern of FIG. 6. FIG. 3 is a block diagram of a second method to form an IBC solar cell. This embodiment performs a first blanket diffusion. The emitter is then etched to expose underlying silicon. The etch mask and the diffusion mask can be the same, although different chemistries are used to etch the oxide mask and to dope the underlying silicon.
The embodiments of FIGS. 2-3 both require a large number of expensive process steps to form an IBC solar cell. While the embodiment of FIG. 3 uses fewer process steps than the embodiment of FIG. 2, it is still complicated and still uses multiple wet steps. For these embodiments of FIGS. 2-3, the addition of an oxide layer is yet another extra process step.
Therefore, there is a need in the art for an improved method of doping for solar cells and, more particularly, an improved method of doping for IBC solar cells using ion implantation.